This invention relates to a method and apparatus for evaluating the performance of devices which generate at least two electrical wave train signals, with each wave train signal varying between first and second states and the two wave train signals normally being out of phase one with the other. One class of such devices are known as quadrature encoders, used to sense the movement of mechanical components such as rotating shafts or laterally moving table or the like, and are common in certain types of machine tools.
Quadrature encoders are well known, commercially available devices. The interested reader is referred to the literature available from suppliers of such devices, such as BEI Electronics of Goleta, Calif. Typically, such devices have one or more optical gratings formed by lines printed or etched onto transparent material such as glass and an associated pair of light source and sensor devices capable of detecting and responding to the lines. The devices are arranged for movement of the grating or source and sensor on movement of a mechanical component, such as the rotation of a shaft or the translation of a table or the like. As a result of movement, at least two wave train signals are generated which are representative of the movement which occurs. The wave train signals can be interpreted by appropriate electronic circuitry and provide position information for control of a mechanical apparatus such as a machine tool. Such technology is, as mentioned, well known and is not within the scope of the present invention.
A problem in the use of such devices which generate wave train signals used to control positioning servo systems and the like is that of assuring that the quality of the signals generated and emitted is sufficiently high to provide assurance that desired or necessary accuracy is maintained. Thus, in the specific case of quadrature encoders, a low quality signal generated by a quadrature encoder will result in inaccurate manufacture by an associated machine tool. As used herein, "low quality" refers to a signal, or more particularly a pair or set of related wave train signals, which lacks a designed and anticipated correlation. In the specific case of a quadrature encoder, the usual design criteria is that the two wave train signals have a phase relation, one to the other, of ninety degrees. Departure from that criteria is a degradation which, should it extend into an undesirably low range, renders the device unacceptable. More particularly, one way of expressing the criteria relates the rate of signal generation to mechanical movement as a frequency response for an encoder, with a typical anticipated response being on the order of 100 kilohertz. As the phase shift between signals approaches zero, the frequency response decreases. For example, should the design criteria a ninety degree phase shift degrade to a forty five degree shift, then the effective response of the encoder is reduced by fifty percent to 50 kilohertz (in the example given here).
While phase measuring instruments have been proposed heretofore, such as in Nessler U.S. Pat. No. 3,906,361, no such instrument has been proposed which addresses the determination of the quality of signal generated by a device of the general type described above.